Just like eBooks, you can never really have enough eReaders. Ok, maybe that isn't true. The eReader app for the iPhone allows you to load up different eBooks on to your device and read them directly from it. Of course there needs to be all sorts of different options available to make reading from your iPhone more comfortable, so eReader made sure not to leave anything out.
eReader offers about the same options as Stanza does, but many of them are located inside of the app itself instead of requiring you to go to the iPhone Settings app. This makes things easier, but not by a lot, as you won't really be needing to change any of those options from the start.
eReader also allows you to download eBooks directly to your iPhone, but you'll need to either register for their service through Safari initially or you'll have to set it up to work with another eBook site that it can successfully interface with. Both of these options seem much more difficult to handle, especially when Stanza allows you to upload them so easily from your computer.
So while eReader is a great app for reading eBooks on your iPhone, it doesn't add anything that Stanza hasn't done so far. While that doesn't give eReader any marks down, it doesn't mean you should pick it over Stanza. It really becomes a matter of preference overall.